In many physical applications, there is a need for sealingly closing containers of substantially plastic material by means of an oblate or a lid of substantially plastic material. In particular for containers intended for the long-term storage of foods, extreme demands are placed on the closure technology, since unsealed closures may entail that, after a time, the contents of the containers will be tainted, with the consequential risk of poisoning if the contents are consumed. In certain physical applications, pasteurization of the filled container takes place in conjunction with the filling operation, which places extremely high demands on the strength of the closure, because of the combination of high temperature and high interior pressure which occurs during the pasteurization.
In order for a satisfactory and tight closure to be obtained, it is, in such instance, necessary that a substantially uniform abutment pressure be applied between those surfaces in which lid and container body are united with one another. In practice, it has proved that even at relatively slight variations in the pressure in different parts of the region of the intended joint, unacceptable sealing in the closure will occur. The requirement of uniform pressure distribution described here applies, for example, in hot glue sealing, using hot melt adhesive, and particularly in physical applications in which the lid is welded against the opening edge of the container.
Containers of plastic material for foods are often produced from laminate material, there being on occasions as many as 8-10 layers in the laminate. One problem which occurs in thick and/or laminated plastic material is that the thickness of the material occasionally varies. In such an event, it is generally at the opening portion of the container where such thickness variations occur, in which instance for example an opening flange will, as a rule, have in its circumferential direction, only one thicker portion and only one thinner portion which gradually and continually merge into one another. The thickness variation described in the foregoing particularly occurs in the opening flange of containers produced by thermoforming.
In certain other physical applications in which the material thickness in the anchorage regions of the lid and the opening portion of the container body is great, hot welding is not a practical proposition, since such a welding technology would entail low manufacturing capacity output because of the good thermo-insulation properties of the plastic material. Consequently, attention has recently been directed at the possibility of employing so-called ultrasonic welding for fixing the lid to the opening portion of the container. It is obvious to the person skilled in this art that securement of the lid by means of ultrasonic welding can also be employed in physical applications in which the lid and/or opening portion in the anchorage region consists of thin material. However, it generally applies that, on the employment of ultrasonic welding, extraordinarily uniform distribution of the abutment pressure is required in those surfaces where the weld is formed in order that a tight and continuous joint be obtained in the weld region.
In ultrasonic welding, one of the components (lid or opening portion, respectively) is generally designed with at least one welding indication in the form of a cam-like ridge. In such instance, it is important that this ridge which, for example, is placed on the flange of the container and/or the lid, abuts with constant pressure against the anchorage region of the lid and/or the container in all of its parts, in order that the connection in the weld be tight and complete. To achieve the constant pressure during the circumferential welding, attempts have been made in the art to employ a resilient support. In such instance, it has, granted, proved possible to attain a certain degree of compensation for any possible misalignments which may be present in the structure, and which entail that the support on which the opening flange of the container body rests makes an angle with that plane made by the abutment surface of the welding electrode (the sonotrode) of the welding device, but because of the characteristics of the springs involved, an unacceptable variation of the abutment pressure may occasionally occur in the region of the contemplated joint. The same applies if the springs are replaced by an elastically yieldable material, for example a pad of rubber.
Attempts have also been made to render the structures highly rigid so as to avoid these problems. However, also in such physical applications it has proved impossible to achieve such strict tolerances that the requisite correspondence of the abutment surfaces of the support and the welding device is realized, and thereby the desired weld quality. Nor can any compensation for unevenness in material thickness be achieved using a technology geared to the employment of rigid structures.
It has proved, in ultrasonic welding, that the distance between the welding surface of the welding device and the back-up may only vary by at most 0.1 mm in order that the desired quality in the joint between the lid and the opening portion can be achieved. This limit of 0.1 mm also includes calculated variation in thickness of both the lid and the opening flange. In injection molded lids, it is possible to attain a deviation of at most 0.05 mm, but in vacuum molding, the maximum deviation is larger. If the container body is injection molded, there will thus be obtained, for the opening portion thereof, also a maximum thickness variation of 0.05 mm, which entails that the maximum permitted deviation of 0.1 mm is "consumed" by the plastic material. Hence, even in a substantially completely matching alignment of the plane of abutment of the welding device and the support (for supporting the container body which is to be sealed) there is a risk that an unacceptable joint will be created. In order to achieve completely satisfactory quality of the joint, there is thus required, in ultrasonic welding, a technology which compensates both for differences in the thinner and thicker portions of the opening flange for differences in thinner and thicker portions of the lid, and/or for misalignments in the equipment.